Digital signal processing (DSP) systems usually have an analog-to-digital converter as an input link from the analog domain. An analog-to-digital converter is always assumed to provide a digital signal based on the input level. For a given peak-to-peak input of V.sub.pp, an A/D converter generates a given number of codes at its output port. Consequently, the input-to-output linearity of an A/D converter is very important.
Currently available high priced analog-to-digital converters have one or more linearity controls. These controls allow the user to adjust the output for a given input (e.g., either 50%, or 25% and 75%) level to obtain a linear output. Frequently however, there is some non-linearity in the intermediate input levels.
For commercial grades of consumer appliances using mass production quality analog-to-digital converters such as video signal products, it is imperative that the degree of non-linearity of the analog-to-digital converter be kept under control. For example, in deghoster circuits, the ghost parameters may be computed from a transition in a given region; by way of illustration, a vertical synchronization signal (-40 to 0 IRE) or 0.fwdarw.70 IRE broadcast television (i.e., a BTA) signal. Then, the entire -40 to .apprxeq.120 IRE video signal is "deghosted" based on these parameters. Thus, for reliable deghosting performance, the "assumed" linearity of the signal is extremely important.
Earlier efforts by others to obtain particular output characteristics from analog-to-digital converters have tended toward two different approaches. One approach used an expensive, high-quality analog-to-digital converter designed to include adjustable gain and phase stages. Prior to use, each of the adjustable stages would necessarily be "tuned" to give optimal output characteristics. Cost of the converter, and the time consumed in making the adjustments, are two problems attendant to the first approach. Consequently, the use of such converters is limited to customized, expensive applications.
The second approach, as represented by Kimura, U.S. Pat. No. 4,764,751, applies the output signal "z" from a non-linear analog-to-digital conversion circuit to a look-up table memory containing straight line conversion data, in an effort to obtain compensated digital values "z'" stored in the look-up table, and to thereby provide a conversion exhibiting a particular non-linear (e.g., logarithmic) characteristic. Several stages are coupled to the input port of the non-linear analog-to-digital conversion circuit to adjust offset and gain of the analog signals being applied to the conversion circuit. This approach however, requires measurement of conversion characteristic data of the non-linear analog-to-digital converter, calibration of the conversion characteristic data curve at an origin and at the maximum output value (e.g., 2.sup.N -1) as part of a normalization and curve-fitting step, and calculation of compensated digital values corresponding to the level of the input signal in terms of reference data and the conversion characteristic data. Consequently, because this approach necessarily demands reliance upon offset and gain adjustment stages, the use of a specialized digital-to-analog converters, and measurement of the conversion characteristic data of the converters, it is unsuitable for use with mass production quality analog-to-digital converters. Moreover, this approach neither recognizes, nor addresses the problem of improving the linearity of conversion by linear digital-to-analog converters at values intermediate to the two extremes of the range of conversion.